Mutations in voltage-gated sodium channels have been associated with several types of human epilepsy, including Genetic (Generalized) Epilepsy with Febrile Seizure Plus (GEFS+) and Dravet Syndrome. Within these genetic epilepsies, there is variable penetrance and expressivity of the clinical phenotype, suggesting a role for genetic modifiers. We have developed mouse models with mutations in voltage-gated sodium channels and seizure-related phenotypes with different underlying mechanisms. Scn2aQ54 transgenic mice have a gain-of-function mutation that results in spontaneous, adult-onset partial motor seizures and models features of GEFS+. Heterozygous Scn1a+/- null mice are a model of Dravet Syndrome, a severe, infant-onset epilepsy with progressive worsening accompanied by psychomotor regression. A common feature of these mouse models is that epilepsy severity varies depending on the genetic strain background, suggesting that genetic modifiers influence the phenotype. Scn2aQ54 mice on the resistant C57BL/6J background have delayed onset, decreased severity and improved survival compared to the susceptible (C57BL/6J x SJL/J)F1 background. Conversely, the epilepsy phenotype of Scn1a+/- mice is more severe on the C57BL/6J background, while they have delayed onset and improved survival on the 129S6/SvEvTac strain background. Based on these observations, we hypothesize that multiple genetic modifiers act to influence penetrance and expressivity of the primary epilepsy mutation. Previously we mapped two modifier loci that are responsible for the strain difference in Scn2aQ54 mice: Moe1 (modifier of epilepsy 1) on Chromosome 11 and Moe2 on Chromosome 19. During the previous funding period we performed fine mapping and candidate gene analysis of the Moe1 region, identified the Moe2 modifier gene, and identified additional modifier loci that influence the Scn2aQ54 phenotype. We propose to continue our analysis of epilepsy modifiers using the Scn2aQ54 and Scn1a+/- mouse models. First, we will identify the responsible genes at Scn2aQ54 modifier loci. Next, we will determine the molecular basis of the Moe2 modifier effect. Finally, we will map genetic modifier loci in the Scn1a+/- mouse model and perform transcriptome analysis via RNA-seq for accelerated identification of candidate genes. The major goal of our studies is to identify and characterize modifier genes that influence epilepsy susceptibility and severity. These genes are likely to contribute to common epilepsy syndromes with more complex genetics. Identification of epilepsy modifier genes will provide insight into the basic biology of epileptogenesis and may identify novel therapeutic targets for the treatment of human patients.